A particular field of application of the invention is making composite material parts constituted by a fiber preform which is densified by a matrix by chemical vapor infiltration. The preform or substrate is a texture or fabric formed on the basis of fibers, e.g. a cloth or a felt. In order to enable the matrix-constituting material to infiltrate into the core of the preform, the preform is placed in an oven at a predetermined temperature and a flow of gas is injected into the enclosure under predetermined conditions of pressure and flow rate. The matrix material is obtained by one of the components in the flow of gas decomposing or by a plurality of components in the flow of gas reacting on coming into contact with the preform. Chemical vapor infiltration methods using carbon or ceramics (carbides, nitrides, or refractory oxides, for example) are well known. Reference may be made, in particular, to the following documents: FR-A-2 189 207, FR-A-2 401 888, and EP 0 085 601.
Chemical vapor infiltration is a lengthy process (several tens of hours) and it is often performed in a plurality of successive cycles.
A drawback of the chemical vapor infiltration methods currently in use lies in a deposition gradient of the matrix-constituting material within the substrate. More precisely, the thickness of the deposit falls off perceptibly between the surface of the substrate and the interior of the substrate, and the greater the speed of infiltration, the greater this effect.
Because of this "excess deposition" on the surface, the pores in the substrate end up by being closed at the surface long before the core has been completely densified.
It is then frequently necessary to machine the substrate to give the flow of gas new access to the internal pores thereof. To do this, the incompletely densified part must be allowed to cool so that it can be removed from the furnace, and so that the surface deposit can be removed by machining, prior to returning the part to the furnace and bringing it back up to the desired temperature. Operations of infiltration followed by machining are commonly repeated several times before the desired degree of densification is achieved.
Nevertheless, in the end, densification is inevitably greater in the vicinity of the surface than in the core of the substrate where residual pores remain.
In addition, the surface deposit that results from the impact of the gas flow on the substrate sometimes presents a microstructure or crystallinity that is different from that of the deposit in the core of the substrate.
Thus, an object of the present invention is to provide a method whereby the deposition gradient can be minimized, thereby making it possible to achieve densification that is more uniform, more regular, and more complete.